It is an unfortunate but common occurrence for drill strings to become stuck or lodged in earth boreholes. Sometimes, skillful manipulation of the drill string and drilling fluid flow controls at the earth surface by the driller frees the stuck string without damage. The presence of jars on the drill string to administer axial shock to the stuck portion of the drill string determines the outcome of the efforts. Jars included in the assembly of the drill string must behave as a normal drill string component until, and if, needed.
Fishing jars are used only after a drill string length is left in the well from previous drilling operations. In this case, the fishing jar does not have to withstand the rigors of normal drilling, and certain design liberties can be taken with durability and fluid flow facilities. The fishing jar will often be fitted with a "fishing" device on the lower end to aid in reattaching the fishing string to the lost drill string length. Drilling jars in common use have complicated and costly construction features. Also, there is a continuing problem of radial space within the jar. The result is a flow-through fluid channel too small for normal survey and other gear to be lowered along the drill string bore to the drill head region. One solution is to use hydraulic jars, which have construction features favoring a larger flow-through bore. Problems arise in hot holes with hydraulic jars, and alternatives are needed. The apparatus of this invention addresses those problems.